A variety of metrology devices are used with machines, such as machine tools or robotic machines, to discover errors in positioning of a moveable device, e.g., a tool that interacts with a workpiece. For example, in U.S. Pat. No. 5,489,168 a system is disclosed that is permanently attached to a six-axis machine tool to verify position of the tool being used, independently of the sensor system on the machine's actuator legs. This allows an operator or controller to determine errors in the positions indicated to the machine's control system, and to compensate for those errors Conventionally, the accuracy of a machine, such as a machine tool, has been verified by machining sample parts that are then mounted to a coordinate measuring machine (CMM) which probes select locations on the workpiece to determine deviation from the desired part parameters. If errors exist, the machine tool can be adjusted to compensate for those errors. Many other machines, such as robotic arms, also require verification of positional accuracy of the tool or other object to ensure precision and accuracy of operation. Some of these machines are able to move an object with six degrees of freedom as defined by the standard linear axes, x, y and z as well as the standard rotational axes a, b and c. Therefore, a six-axis metrology device is necessary to verify the accuracy of the machines movement along all six axes.
In some situations, a CMM having an arm constructed of pivotable and rotatable serial links is used to determine errors in positioning of, for instance a tool. Sensors are used to indicate the movement of each serial link relative to the next adjacent link, and the combination of signals can be used to track the movement along all six axes of one end of the arm relative to a datum at its opposite end. One problem with this type of system is that any errors in the CMM itself are additive due to the serial linking of moveable components. It would be advantageous to have the sensors connected in parallel rather than serially to provide an averaging of errors rather than an adding of errors in the device itself. Such a device could be used to verify the accuracy of the CMM arm or, in appropriate cases, to replace the CMM arm.
In many applications, it also would be advantageous to have a six-axis metrology device that could be connected to the machine to discover any errors in the positioning of the machine's moveable component, e.g., tool and then disconnected and removed prior to operation of the machine. In the case of a machine tool, for example, this would permit the accuracy of the machine and its control system to be tested prior to machining sample parts for verification of dimensions on a CMM. It would also obviate the need for constructing a permanent metrology device with each machine.
Furthermore, regardless of whether the metrology device is permanently or removably mounted to the machine being tested, telescopic sensor legs, such as those disclosed in the '168 patent, are sometimes vulnerable to static friction. The inherent nature of telescopic components requires that a certain amount of stick, sometimes known as stiction, must be overcome when movement first occurs. Some precise measurement operations, particularly in applications where the movement of the metrology device is repeatedly started and stopped, would benefit from reduced stiction.
The present invention addresses the foregoing drawbacks of existing metrology devices and methods of determining positional errors in machines.